This invention relates generally to the manufacture of enhanced tubes and more particularly to a method of and an apparatus for measuring the pore size in an externally enhanced evaporator tube.
In an evaporator of certain refrigeration systems a fluid to be cooled is passed through heat transfer tubing while refrigerant in contact with the exterior of the tubing changes state from a liquid to a vapor by absorbing heat from the fluid within the tubing. The external and internal configuration of the tubing are important in determining the overall heat transfer characteristics of the tubing. For example, it is known that one of the most effective ways of transferring heat from the fluid within the tube to the boiling refrigerant surrounding the tube is through the mechanism of nucleate boiling.
It has been theorized that the provision of vapor entrapment sites or cavities on a heat transfer surface cause nucleate boiling. According to this theory the vapor trapped in the cavities forms the nucleus of a bubble, at or slightly above the saturation temperature, and the bubble increases in volume as heat is added until surface tension is overcome and a vapor bubble breaks free from the heat transfer surface. As the vapor bubble leaves the heat transfer surface, liquid enters the vacated volume trapping the remaining vapor and another bubble is formed. The continual bubble formation together with the convection effect of the bubbles traveling through and mixing the boundary layer of superheated refrigerant, which covers the vapor entrapment sites, results in improved heat transfer. A heat exchange surface having a number of discrete artificial nucleation sites is disclosed in U.S. Pat. No. 3,301,314.
It is known that a vapor entrapment site or cavity produces stable bubble columns when it is of the re-entrant type. In this context, a re-entrant vapor entrapment site is defined as a cavity or groove in which the size of the surface pore or gap is smaller than the subsurface cavity or subsurface groove. Heat transfer tubes having re-entrant type grooves are disclosed in U.S. Pat. Nos. 3,696,861 and 3,768,290.
It has been discovered that an excessive influx of liquid from the surroundings can flood or deactivate a re-entrant type vapor entrapment site. However, a heat transfer surface having subsurface channels communicating with the surroundings through surface openings or pores having a specified "opening ratio" have been found to provide good heat transfer and prevent flooding of the vapor entrapment site or subsurface channel.
In regard to the interior surface configuration of a heat transfer tube, it is known that providing an internal rib on the tube may enhance the heat transfer characteristics of the tube due to the increased turbulence of the fluid flowing through the ribbed tube.
As disclosed in U.S. Pat. Nos. 4,425,696 and 4,438,807 assigned to the present assignee and incorporated by reference herein, an internally and externally enhanced heat transfer tube, having an internal rib and an external helical fin (creating a subsurface channel) communicating with the surrounding liquid through surface openings (pores) is manufactured by a single pass process with a tube finning and rolling machine. According to the disclosed process a grooved mandrel is placed inside an unformed tube and a tool arbor having a tool gang thereon is rolled over the external surface of the tube. The unformed tube is pressed against the mandrel to form at least one internal rib on the internal surface of the tube. Simultaneously, at least one external fin convolution is formed on the external surface of the tube by finning discs on the tool gang. The external fin convolutions form subsurface channels therebetween. The external fin convolutions also have depressed sections above the internal rib where the tube is forced into the grooves of the mandrel to form the rib. A smooth roller-like disc on the tool arbor is rolled over the external surface of the tube after the external fin convolution is formed. The smooth roller-like disc is designed to bend over the tip portion of the external fin so that it touches the adjacent fin convolution and forms an enclosed subsurface channel. However, the tip portion of the depressed sections of the external fin, which are located above the internal rib, are also bent over but do not touch the adjacent convolutions, thereby forming pores which provide fluid communication between the fluid surrounding the tube and the subsurface channels.
The performance of the foregoing tube is critically dependent upon the external enhancement of the tube. It is therefore important to maintain a consistent subsurface channel size and pore size during the manufacturing process. Normal variations in subsurface channel size and surface pore size do occur, however, due to tool wear, material variations in the tube, dimensional variations in the tube lengths, and machine tolerances. In order to account for these variables and to maintain a consistent pore size, it is necessary to measure the pore size on each tube produced and adjust the finning machine to maintain the correct subsurface channel and pore sizes. However, the prior methods of checking the pore size in an enhanced tube were very laborious and expensive processes, and could not be used in a manufacturing process. For example, one method was to have an operator randomly select a manufactured tube and optically check the pore size of the selected tube under a microscope. Another method was to take a photograph of a tube and using an image analyzer compare the area of the pores in a selected area to the area of the pores in a reference photograph. However, these methods were time consuming and did not provide the quality and quantity of tubes necessary for a manufacturing process.
Thus, there was a clear need for a method and apparatus for measuring the size of the surface pores in an enhanced tube that would, to a large extent, overcome the inadequacies that have characterized the prior art.